xii

FIG.

LIST OF ILLUSTRATIONS

6. Sections of Alpine valley and Scotch glens .

7. Old Lake-Terraces of the Salt-Lake area of Utah

8. The island of Vulcano in eruption

9. Small Cones thrown up on Etna

10. Section of a Volcano

PAGE

83

139

155

159

161

11. Floating Scoriæ above the Submarine Eruption off Pantelleria . 163

12. Microscopic section of Obsidian

165

13. Microscopic section of Basalt

167

14. The Puy de Lassolas and the Puy de la Vache

176

15. Geological Map, showing Lava-flows descending from Puys in

16. Granite Pinnacles, Mourne Mountains

17. Unconformable Junction, with Overlap and Overstep

18. Internal cast of a Cerithium, Portland Stone

19. a. Pecten Beaveri; b. Pecten cinctus; c. Pecten islandicus 20. Trilobites

21. Section across the Surrey Hills

22. Section illustrating Variation of Width of Outcrop

23. Sand with Spicules of Siliceous Sponges, Hythe Beds, Surrey 24. Ancyloceras gigas

28. Section illustrating Fan-Structure in a Mountain-Chain

284

29. Section illustrating Recumbent Folds

284

30. Section across a Fault

285

286

31. Plan of a Fault in inclined Strata

32. Ridges and Valleys formed by Folds in the Jura Mountains

33. Recumbent Synclinal and Folding on the Windgälle

291

297

OPEN-AIR STUDIES

CHAPTER I

THE MATERIALS OF THE EARTH

WHEN we were children, as soon as we could think and ask questions, most of us tried to find out two important matters-how to make things, and what things are made of. The little boy who cut open his drum to see where the sound came from was not so entirely foolish as is sometimes represented, for he at least made an interesting and valuable experiment. He learnt in this way that a drum was best constructed by stretching two thin skins opposite to one another, and shutting in a quantity of air between them. He was not satisfied with the mere showy outsides of things, or with the royal arms of Great Britain and Ireland painted on the wooden barrel of the drum. He felt that the sound, which was the great feature of the instrument, must have a cause; and he set to work to find out something about it for himself. Probably he only got as far as seeing how the drum was made, and had to ask his father for the reason of this particular arrangement. But that is what every one has to do, and why no great discovery is likely to be of use to us unless we know what other people have previously discovered. In the same way, we cannot write a passably useful book unless we are near the 50,000 volumes of a well-stocked public library. Our knowledge in this world does not go on by jumps and bounds, but by a study of what others have seen and done, and of the records of their failure or success. Even if, after a long series of experiments, we have to leave our old drums damaged and cut open, we may soon have wit enough to make newer and better ones for the service of ourselves and our descendants.

A

In early times, it was most important that man should find out how to make things. He had to build shelters against the rain; he had to find weapons with which to combat the wild beasts; he had to carve out gourds into vessels in which liquids could be carried. But all these things were by no means easy, and required a good deal of observation. Nature was already doing many wonderful things in the great world round about, and it was well to watch, and even to imitate, these closely. After all, whatever was required by man had to be made out of natural materials. One result of this struggle with difficulties was that early man took to studying the stones beneath his feet. Some of his earliest instruments were made of stone; and we know that many savage tribes have only given up the use of stone axes and stone arrow-heads since the arrival of European traders on their coasts. Primitive peoples would soon observe that some kinds of stone break more easily than others, that some kinds can be chipped in almost any direction so as to give a sharp cutting edge, and that some, again, are far superior in hardness or durability to the rest. It became important to search among the rocks for materials suited to each special purpose; and thus began, perhaps, the first researches into the structure of the world around us.

When native metals were discovered, and when, moreover, it was found that valuable substances could be extracted by heat from stones utterly unlike them in appearance, the study of the earth became a much more complicated matter; and at last a few thoughtful persons took to inquiring how such-and-such materials, used by man in the manufacture of common things, came themselves to be made in the heart of the old solid earth. Then began the science of Geology, the systematic study of the way in which the world is built up, and of the natural changes which go on within it and upon it.

Even the earliest men must have seen that the earth

changed around them. Floods came down, and covered smiling landscapes with sand and stones from distant hills; volcanoes broke out, and heaped ashes and lava upon the surface; the sea worked against the land, and washed the coast steadily away; while in other places tawny rivers wandered through the plains and thrust their burden of mud in long banks far to seaward.

Thus the earth changed its surface, growing at one place, decaying away at another; and men began to realise that it had a history, beside which their own lives were short indeed. And yet it is only in the last hundred years or so that we have been able to read this history at all clearly, and even now we understand only that end of it which lies nearest to ourselves.

Before we go out and try to read a page or two of this history, we must learn something of the materials with which we have to deal. Of what, then, is the earth actually made?

As a matter of fact, we know only a very small part of the vast globe on which we live. The centre of the earth lies 4000 miles beneath us, and our mines and borings penetrate the great mass to a depth of only about a mile. ~But, as we shall see later, the outer layers of the earth have become wrinkled and folded, so that rocks which once lay far below the surface have been brought up within our reach. Hence in some places, immediately under the loose soil, we may find materials which were formerly ten or fifteen miles lower down; and this enables us to say that we have some idea of the constitution of the globe to a depth of fifteen miles from the surface. It is not much, this mere outer shell, fifteen miles thick all round the earth; but it is all we shall have to deal with, and we speak of this accessible region as the crust of the earth.

This solid crust is made of rocks. Any workman in a quarry will tell us, for instance, that granite is a rock, or that sandstone is a rock. But, when we examine these two materials, we see that they are made up of distinct particles which themselves are worth inquiring into. The granite has clear little lumps, like glass, in it; and dull white or pink bodies, which seem to break across in a regular manner, with smooth or step-like surfaces; and soft flaky things which reflect the light brilliantly, and which can be pulled away, or dug out with a knife, in the form of little shining plates. The sandstone, on the other hand, is made of practically one kind of material throughout, in the form of a number of hard and somewhat rounded grains.

In each case we call these constituents, having characters of their own, the minerals which compose the rock. The granite consists of at least three distinct minerals, the sand

stone of only one. Now, if we break up a rock, and select a number of fragments of any one kind of mineral, a chemist will show us that this mineral is itself made up of certain chemical substances, which he can separate and accurately determine. He will be able to find out in what proportions these chemical substances are present, and will thus give us an account of the chemical constitution of the mineral.

But what about these various substances? They may be stated by our friendly chemist to be chemical compounds, and these can be further split up into their elements. Or, to take the simplest case, our mineral may possibly consist of merely one element.

Here we may remind ourselves of some of our chemical notions. We must go back, indeed, to the foundations of several sciences for the correct understanding of our rocks, and this first chapter is going to be a serious one, before we can fairly get out into the country and the open air. But we must remember that all the things with which we shall deal are quite natural, all come from the earth itself that stretches round us; and those parts of chemistry and physics which are possibly a little dull to us seem so merely because we cannot handle or directly observe the things about which we are being told. If we could see the tiny "atoms moving in the "molecule" of a substance, we should probably be as delighted and as fascinated as if we were sitting on a mountain-peak and watching the great balance of the stars.

A chemical element is a substance that cannot be split up, by any means at present in our power, into any other substances with different properties from those by which it is itself characterised. It is thus the simplest kind of material known to us. Gold, silver, copper, and iron are elements; sulphur is an element; so also is carbon, well known to us in its commonest form, the "black-lead" of our pencils. There are now about seventy elements known, and others, at present doubtful, are likely to be added to the list.

The ancients regarded the earth as an element in itself, something existing as a foundation for other things, the remaining elements being air, fire, and water. What we

now know as elements were often treated in earlier times as varied forms of the same fundamental material; but all